GAZA, Aug 3 (Reuters) - Gaza's Islamist Hamas government has freed a militant group's leader who had been detained for 17 months, a source from a Jihadist Salafi group said on Friday.

Hisham Al-Saedni, also known as Abu Al-Waleed Al-Maqdissi, was freed late on Thursday, the source said. Hamas declined immediate comment.

Saedni, in his late 50s, is believed to head the Jihadist Salafi group Tawhid and Jihad (One God and Holy War). The group adopts an Islamist ideology shared by al Qaeda, and sources said that Saedni himself joined al Qaeda in Iraq at the beginning of the U.S.-led invasion in 2003.

Hamas had accused Saedni of attempting to disrupt public order in the Gaza Strip and arrested him on March 4, 2011. Hamas seized the Gaza Strip from Palestinian President Mahmoud Abbas's allies in 2007 following a brief civil war.

Members identifying themselves with Tawhid and Jihad kidnapped and killed a pro-Palestinian Italian activist, Vittorio Arrigoni, last year, in an apparent attempt to secure the release of Saedni.

A number of Jihadist Salafi groups have surfaced in Gaza in recent years. Unlike Hamas, they endorse an ideology of global Jihad and some accuse Hamas of failing to implement Islamic laws in the coastal enclave. (Reporting by Nidal Almughrabi; Editing by Crispian Balmer)